Control applications, such as heating, ventilation, and air conditioning (HVAC) temperature control, and control of fluid levels, often involve the sensing of external isolated signals. Typically, the signals that are sensed include contact closures, DC voltages, and AC 60 Hz voltages with unknown voltage references. The signals could be connected to ground or to other potentials that can be different from the reference potential of the circuits that are attempting to evaluate their status.
FIG. 5A is a schematic diagram illustrating a conventional circuit for sensing an external direct current (DC) or alternating current (AC) voltage. FIG. 5B is a schematic diagram illustrating a conventional circuit for sensing an isolated contact closure. In these conventional approaches, different circuits are required for sensing voltages and for sensing contact closures. Requiring separate circuits to handle these different external signals adds complexity and cost to the overall design of a device that needs to implement both functionalities. In addition, requiring the sensing circuit or isolated power supply to deliver 10 milliamperes (ma) of optical diode current may be difficult in low power designs.